This invention relates to a front and rear wheel steering device for variably controlling the steering angle of rear wheels relative to the steering angle of front wheels.
The present inventors have previously proposed a steering device for a vehicle which steers the rear wheels in relation with the steering of the front wheels according to vehicle speed in copending U.S. patent application Ser. Nos. 822,000, 822,293, 821,998, 822,008, 822,010 and 822,043 which were filed on Jan. 24, 1986, and assigned to the same assignee. According to these devices, rear wheels are generally steered in the same phase relationship or none at all in high speed range and are steered in the opposite phase relationship in low speed range. If desired, the mathematical function of the vehicle speed for the steering angle of the rear wheels can be modified according to the vehicle acceleration or manually, or can even be manually fixed.
As a result, the minimum angle of turning and the inner radius difference of the vehicle are both drastically reduced and the maneuverability of the vehicle, particularly in low speed range, in driving the vehicle into a garage, driving the car through narrow and crooked alleys and making a U-turn, is substantially improved with the additional advantage of improving the dynamic lateral response of the vehicle in high speed range, for instance in changing driving lanes.
Generally speaking, since the driveability of a vehicle depends on the vertical load acting between the tires and the road surface, the friction coefficient of the road surface, the pneumatic pressure of the tires, the kind of the tires and so on, and the relationship of the steering angle and/or the vehicle speed on the yaw rate is generally non-linear, it is extremely difficult to evaluate the driveability of a vehicle in design stage. On the other hand, if the rear wheels are steered in addition to the front wheels, a controllable variable is added to the system and it may be possible to better evaluate or control the driveability of the vehicle by changing the steering angle ratio of the rear wheels relative to the front wheels. Based upon such a recognition of the inventors, a primary object of the present invention is to provide a front and rear wheel steering device for vehicle which can improve the driveability of the vehicle by appropriately controlling the steering angle ratio of the rear wheels relative to the front wheels.